High grasshopper numbers possible

News
Apr 30, 2010
by DTN
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Entomologists are concerned about high numbers of pasture-eating grasshoppers this year, though the extent of the infestation won’t be known until late May or early June, according to Bob Wright, entomologist at the University of Nebraska-Lincoln’s (UNL) Institute of Agriculture and Natural Resources.

USDA surveys indicate a potential for heavy infestations. “Although we can’t target a particular calendar date, the weather between now and June can influence how quickly grasshoppers grow,” Wright said.

USDA surveys rangeland in western and central Nebraska in the fall to measure the number of adult grasshoppers active at the end of the year. Survey numbers give an index of how many eggs have been laid. “That is why we think there is a high risk of infestation, based on the high numbers of grasshoppers last fall,” Wright said.

The surveys are of rangeland-plant-feeding grasshoppers only, not the crop-damaging type or the other 100-some types of grasshoppers in Nebraska.

Wright said grasshopper populations fluctuate in cycles. Numbers have been high in the past few years. Adding to the buildup concern is that there were no weather events at the end of 2009 that would cause the population to drop off.

“An early freeze can reduce populations, but we didn’t have that last year,” he said. “There were plenty of grasshoppers surviving through the fall.”

Two factors will largely determine the extent of the grasshopper population this growing season: food availability and rainfall.

Rainy weather can allow pasture plants to outgrow the damage from feeding grasshoppers. Grass grows less in dry weather, making grasshopper damage more extensive.

The insects are cold blooded, and have little fat reserves when they hatch, making food a necessity. If the weather is 60 degrees Fahrenheit or lower at hatching time, the grasshoppers are unable to move and can’t feed enough to survive. That will limit populations, he said.

Wet weather also can encourage diseases that attack grasshoppers. High insect populations make it more likely that diseases spread from grasshopper to grasshopper, similar to the way humans transmit the flu.

“High populations competing for food and under stress can increase the spread of disease and limit populations,” Wright said. “That is one reason why after a couple years of high infestation, numbers decline from overcrowding. It won’t totally eliminate them, but it can decrease numbers.” Infestation possibilities in Nebraska The UNL researchers have developed a map of Nebraska counties most likely to experience grasshopper infestations (http:// entomology.unl.edu/).

Although there is some risk into Knox County, the risk areas end almost parallel with the eastern borders of Holt, Wheeler, Greeley and Howard counties, then along the eastern borders of Buffalo, Kearney and Franklin counties.

The areas of highest infestation, however, are in the central and western parts of Nebraska, from the eastern borders of Loup, Custer and Dawson counties west and into the Panhandle.

“Basically, the western half of Nebraska has a moderate to high potential for grasshopper problems in rangeland areas,” Wright said.

There is little concern for infestations moving eastward, Wright said, as grasshoppers typically do not move great distances.

“We do not get the types of grasshoppers that move in swarms like in Africa or the U.S. in the 1800s,” he said. “They can move several miles, or possibly from field to field, but not county to county. They cannot travel from Scottsbluff to Lincoln.”

Concern in other states

Other areas with high potential for grasshopper damage include eastern Wyoming, western South Dakota and parts of Colorado.

According to USDA’s 2010 Rangeland Grasshopper Hazard map, the areas for greatest concern are: southwestern South Dakota, northeastern Wyoming, and much of Montana. There are smaller areas of concern in the southern and western areas of Idaho, as well as eastern areas in Oregon and Washington.

The USDA map, at www.aphis.usda.gov/, also shows a sizeable area of concern in western Kansas, as well as eastern Colorado and western Oklahoma.

Advice for ranchers

Wright said that ranchers need to check their land in late May-early June as grasshoppers hatch out.

“If they do need to control grasshoppers with insecticide, it is much more effective when grasshoppers are small, before they grow to adulthood.”

More information on grasshopper controls can be found at the UNL Web site: http://entomology.unl.edu/. — Cheryl Anderson, DTN


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